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PNW CONSERVATION TILLAGE HANDBOOK SERIES
Chapter 5 - Weed Control, No. 7, Spring 1987


Russian Thistle Control in a Wheat-Fallow Rotation

Roger Veseth

Russian thistle is an annual broadleaf weed of wide spread economic importance. In the Pacific Northwest, Russian thistle is a serious problem in the lower rainfall, nonirrigated, small grain producing regions in eastern Washington and Oregon, and southwestern Idaho. In Washington alone, there are about 355,000 acres infested with Russian thistle, with the heaviest infestations being in the wheat-fallow regions. It is often the predominant broadleaf weed in these areas. Where infestations occur, it can significantly reduce crop yield and harvest efficiency, and in some cases completely prevent crop harvest. Russian thistle can be a particular problem in spring grains or where fall cereal has sustained winter-kill damage.

Until the past few years, most available broadleaf herbicides provided only poor to fair control of Russian thistle. This was attributed, in part, to the lack of herbicide persistence in the soil. Russian thistle has the capability of germinating through May 1 and has occasionally been reported to germinate later. This is well after broadleaf herbicides are usually applied to wheat in the wheat-fallow region. Control through herbicides could reduce tillage operations, potentially reducing costs. Reduced tillage would also improve soil and water conservation.

The Research Effort

STEEP researchers Frank Young, research agronomist, and David Gealy, plant physiologist, with the USDA Agricultural Research Service at Washington State University in Pullman have been working to develop effective control options for Russian thistle. Their research indicates that soil residual herbicides such as Glean (chlorosulfuron) can provide effective Russian thistle control at labeled application rates. They evaluated several application periods during the wheat-fallow rotation.

Field experiments were conducted in 1981, 1982 and 1983 at the Washington State University Dryland Research Unit near Lind and on a farm near Benton City, west of Richland. Both sites receive similar amounts of precipitation and were on Ritzville silt loam soils. Annual precipitation measured at Lind ranged from 8 inches in 1981 to 13.4 inches in 1983. The 60-year average is 9.4 inches.

Postharvest Burndown with Glean in 1981 (Oregon and Washington Label Only)

Control of Russian thistle shortly after harvest in a wheat-fallow rotation is important to reduce seed production, as well as weed growth and to conserve water. Russian thistle germinates in the summer of the crop season and commonly flowers by harvest time, so timely control is important, Young has found that, after harvest, Russian thistle can produce up to 93 percent of their total dry weight and a single plant can use as much as 68 pounds of water and produce over 17,000 seeds.

Postharvest control of Russian thistle by tillage is often delayed until growers have harvested all their fields and preparations for fall planting have been completed. By this time, Russian thistle plants have grown considerably, have almost completed flowering, and may have begun to form viable seed. Plants at this stage of growth are more difficult to control and will add to the seed reservoir in the soil.

Subsurface tillage implements, such as sweeps, are commonly used for postharvest control of Russian thistle. A sweep will kill small thistle plants and typically leaves about 85 percent of the crop residue on the soil surface. However, if tillage is delayed, or there is a moderate to heavy Russian thistle infestation, a disk is often required to chop the large amount of thistle residue that could otherwise interfere with later operations. Disking commonly buries about 40 percent more residue than sweeps, consequently reducing soil protection. Wind and water erosion, during the following summer fallow season and subsequent wheat crop, can be severe in these low rainfall areas. Rough tillage provides only temporary erosion control in the absence of surface residue.

Postharvest chemical control of weeds can potentially reduce soil and water loss by eliminating postharvest tillage for weed control. This would allow standing stubble to remain overwinter to trap additional water as snow, reduce evaporation loss and soil freezing.

Glean is now labeled in Oregon and Washington for postharvest burndown of Russian thistle (NOT IN IDAHO) 3 to 10 days after harvest in a wheat-fallow rotation. In this study, Young and Gealy found that chlorsulfuron plus surfactant effectively controlled Russian thistle shortly after harvest and could eliminate a primary fall tillage operation. A % ounce/acre rate of Glean as the formulated product, plus 0.25 percent nonionic surfactant (volume/volume basis), controlled 76 percent of the Russian thistle postharvest. Without surfactant, control was reduced to 35 percent. Russian thistle was in the flowering stage with plants 10 to 12 inches tall.

The 1982 Summer-Fallow Season

Residual carryover of the Y3 ounce/acre rate of Glean as formulated product applied as a postharvest bumdown treatment (NOT LABELED IN IDAHO) in fall 1981 resulted in excellent Russian thistle control the following summer-fallow season. Weed control ratings were 98 percent in June, 97 percent in July and 94 percent in September 1982. The researchers point out that a later fall application of Glean for control of new small Russian thistle would provide similar residual control the following summer-fallow season. This application period is labeled in Oregon, Washington and Idaho.

Fall application of Glean could delay the usual March primary tillage operation until Mayor June, and thus reduce the number of rodweeding operations normally required in a stubble-mulch system. A chemical-fallow program could also be continued instead of tillage, depending on the success of maintaining adequate seed zone moisture and cost comparisons with tillage.

An application of % ounce/acre Glean as formulated product in June 1982 provided 89 percent control of Russian thistle through July of the summer-fallow season. However, by September, control decreased to 41 percent, The researchers attributed the decrease in Russian thistle control by the end of the season to regrowth of the treated plants, as well as to germination and establishment of new plants. The dry summer may have reduced the effectiveness of the spring application. Light rain showers amounted to only 0.2 inch in July and 0.04 inch in August. These showers were sufficient to germinate Russian thistle but not sufficient to move the herbicide into the thistle root zone.

With sufficient winter precipitation and the improved efficiency of soil water storage with standing stubble, annual cropping with spring wheat could be considered in place of fallow in some years. After a fall application of Glean, no additional control of Russian thistle may be needed during the recrop spring wheat season. Glean tolerance of the spring wheat variety must be considered, however. In a greenhouse study, the researchers found the variety "Wampum" to be more susceptible to Glean soil residual than ' 'Dirkwin. " In years when summer cropping is not possible, residual control of Russian thistle during the summer-fallow season would allow producers to use a conservation tillage system.

Russian Thistle Control In Spring Wheat, 1983

After a single disking in the spring 1982 fallow season, the research sites at Lind and Benton City were left untilled through the winter. In spring 1983, spring wheat was planted after a harrowing operation. Glean at 72 ounce/acre formulated product was applied in April where no Glean treatment had previously been used. Russian thistle was in the seedling stage, 1 to 2 inches tall, when the herbicide was applied. Above-normal rainfall from May through July totaled 6.4 inches.

Table 1 shows the percent control of Russian thistle with the April 1983 Glean application, evaluated in May, July and August 1983. Residual control of Russian thistle in the 1983 spring wheat crop, from the September 1981 and June 1982 Glean applications are also shown.

Early season suppression of Russian thistle allowed the spring wheat a competitive advantage over the weeds. The September 1981 % ounce/acre formulated product application of Glean gave 69 percent control in May 1983 and 45 percent control by August. With this application, spring wheat yielded 75 percent more than the untreated control. The highest spring wheat yield in 1983 occurred with % ounce/acre formulated product rate applied in June of the 1982 summer-fallow year.

Table 1. Russian thistle control and grain yield of spring wheat in 1983 after single Glean applications in 1981 (fall postharvest), 1982 (spring of fallow season) and 1983 (in growing crop) at Lind and Benton City, WA (Young and Gealy, USDA-ARS, Pullman).

Date of application1 Russian thistle control, 1983  
May July August Grain yield
  (%) (bu/acre)
Untreated control 0 0 0 15.4
Carryover control with 1981 or 1982 applications only
September 1981 69 63 45 27.0
June 1982 74 71 71 29.8
Control with in-crop application only
April 1983 89 81 76 28.7

1Applied at 1/3 ounce/acre of formulated product plus 0.25 percent nonionic surfactant (vol/vol).

Additional Research On Postharvest Burndown

More recent research by Young and Ralph Whitesides, Washington State University research agronomist at Pullman, reaffirmed the value of postharvest burndown application of Glean (OREGON AND WASHINGTON LABEL ONLY) for Russian thistle control in a wheat fallow rotation. The research was conducted in 1983-84 and 1984-85 near Washtucna, WA, in a 9- to 12-inch average annual precipitation area.

Glean applied as a postharvest burndown treatment (NOT LABELED IN IDAHO) at rates of 1/3 and 1/2 ounce/acre of formulated product plus 0.25 percent nonionic surfactant (vol/vol) controlled 73 and 76 percent of the Russian thistle, respectively. Glean application also reduced germination of Russian thistle seed produced by the sprayed plants. In the untreated control, 59 percent of the Russian thistle seed germinated in 14 days. Plants treated with Y3 and % ounce/acre of Glean as formulated product had only 18 and 15 percent germination, respectively. Since Russian thistle seed is short lived in the soil, nearly all germinating the first year, reduction in seed viability as well as seed production can be important in reducing Russian thistle infestations.

Young and Whitesides evaluated the residual control of fall-applied Glean during the following summer-fallow season. In July of the fallow season, 10 months after the September application, the 1/3 and 1/2 ounce/acre rates of Glean as formulated product gave 94 and 95 percent control, respectively, of Russian thistle when compared to the untreated controls.

Conclusion

The researchers concluded that Glean can effectively control Russian thistle and increase wheat yields when applied in the fall after harvest, during the fallow season or in the growing crop. Chemical control of Russian thistle would facilitate the use of conservation tillage systems for improved water storage and soil protection. Fewer, less intensive tillage operations would be required. Consequently a higher surface residue level could be maintained to reduce soil erosion and water loss through the fallow season and after the following winter wheat crop was planted. Reduction in seed production and seed germination would also reduce the weed seed reservoir for future infestations.

Use of Trade Names

Research results are given for information only and are not to be construed as a recommendation for an unregistered use of a pesticide. Always read and follow label instructions carefully. To simplify the information, trade names have been used. Neither endorsement of named products is intended nor criticism implied of similar products not mentioned.

     
 

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